The use of fiber Bragg gratings in components for optical telecommunication systems such as lasers, amplifiers, filters, add-drop multiplexers, wavelength multiplexers/demultiplexers, etc. has been known for some time. A review of the use of fiber Bragg gratings as components of optical telecommunication systems is found for instance in the papers "Lightwave Applications of Fiber Bragg Gratings", by C. R. Giles, Journal of Lightwave Technology, Vol. 15, No. 8, August 1997, pp. 1391 et seq., and "Fiber Gratings in Lasers and Amplifiers", by J. Archambault and S. G. Grubb, ibid., pp. 1379 et seq.
In particular, in applications in wavelength division multiplexing systems it is necessary to have devices capable of separating the various channels. For this purpose it is possible to use gratings of which the reflection spectrum presents a peak that is, insofar as possible, narrow and free of side lobes.
When fiber Bragg gratings are used to make one or both the reflecting elements that delimit a resonant cavity of a component, e.g. a Fabry-Perot cavity laser, to be used in such systems, one encounters problems linked to the cavity length. This length depends, as is well known, on the position of the so-called equivalent mirror plane of the grating. The equivalent mirror plane is the plane where a mirror would have to be positioned in order that a pulse sent by a source and reflected by the mirror returns to the source in the same time the pulse sent into the grating would take to return. The distance between the equivalent mirror plane and the input end of the grating constitutes the equivalent length of the grating. The length of a resonant cavity that makes use of fiber Bragg gratings is therefore represented by the distance between the equivalent mirror plane of the grating and the other reflecting element of the cavity (if only one such element is made by a grating) or between the equivalent mirror planes of the two gratings (if both reflecting elements are made by gratings). Now, if the linewidth of the laser is to be kept limited, the length of the cavity cannot be shorter than a certain minimum length, which is determined by manufacturing requirements; on the other hand, the longer the cavity, the shorter the distance between the modes and hence the harder the separation between the different modes.
The gratings proposed until now have a modulation of the refractive index which, as a function of the length of the grating, presents a symmetrical profile with respect to the central point of the grating. In these symmetrical gratings the equivalent mirror plane is placed substantially at the center of the grating, if the latter is a low-reflecting grating, and is located in a more advanced position towards one end if the grating is a highly reflecting grating. "Low-reflecting" indicates a value of reflectivity such that, when the grating is used as the reflecting element of the cavity, the radiation fraction exiting the cavity is sufficient for practical uses (typically, a reflectivity of the order of 70% in a laser); "highly reflecting" indicates a reflectivity of practically 100% or very close to this value. A highly reflecting grating could be used as one of the reflecting elements of the cavity, thereby reducing its length, provided the other reflecting element presents a sufficiently high transmission factor. In the case of a cavity with only one reflecting element made by a grating, the latter is positioned in correspondence with the output end and the use of a highly reflecting grating under such conditions is clearly inconceivable. In the case of a cavity where both reflecting elements are made by gratings (in the example, the cavity of an all-fiber laser), the use of a highly reflective grating does not solve the problem of obtaining a narrow band with a very reduced length of the cavity, both because the spectral line of those gratings is in any case relatively wide, and because one of the gratings should be a low-reflecting grating and hence would present a high equivalent length.